Is NASA Really on the Verge of Finding Extraterrestrial Life?

When I was a kid, NASA's space programs played a major role in inspiring my interest in science. So it saddens me today to see how the space agency, to justify its share of the federal budget, overstates the case for habitable planets and extraterrestrial life. Such claims from NASA are a staple of Internet journalism, virtually unavoidable if you go online. Unfortunately the best available science, including a new paper by our colleague and astronomer Guillermo Gonzalez, demonstrates that the problem of supporting life on a hypothetical exoplanet is much, much tougher than NASA suggests to the public.

"Sometime in the near future, people will be able to point to a star and say, 'That star has a planet like Earth'," said Sara Seager, professor of planetary science and physics at the Massachusetts Institute of Technology in Cambridge, Massachusetts. "Astronomers think it is very likely that every single star in our Milky Way galaxy has at least one planet."

What is the truth? I asked Dr. Gonzalez, a Senior Fellow with Discovery Institute's Center for Science & Culture, about Seager's claim that every single star in our galaxy will have a planet. He replied, "Yes, obviously an exaggeration. Stars much more metal-poor than the Sun won't have planets. Of course, most stars will have planets if you include planets as small as Mercury."

It's simply not true that every star has planets. Even if most stars do have planets there's little reason to believe that many, or hardly any (if any), of those will be habitable. Seager points to a planet's surface temperature as if that's the main factor that determines habitability. But there are many other conditions that must be satisfied if a planet is to be well suited for life. For example, it might be at just the "right" temperature for liquid water, but if there is no water to being with then the temperature won't do you any good. Indeed, Earth seems to be special in having a large amount of water. There also must be a protective magnetic field, and the planet needs to be in the right place in the galaxy (the "galactic habitable zone") to avoid extreme life-destroying radiation that is found in many regions. There are many other requirements. Suffice to say, Earth has many unique properties that go well beyond its temperature.

The circumstellar habitable zone (CHZ) has served as a unifying concept in astrobiology for several decades, but the broader astrophysical context of habitability (e.g., the origin and distribution of the elements that go into forming planets) requires that we also consider galactic-scale habitability (galactic habitable zone (GHZ)) and cosmic-scale habitability (cosmic habitable age (CHA)).

For example, even the CHZ depends not only upon temperature (or distance from the host star), but also upon other factors, like the amount of UV light received by the planet:

Another constraint on the boundaries of the CHZ comes about when the positive and negative effects of the UV radiation from the host star are included. The inner boundary of the "UV-CHZ" is set by the maximum UV flux that DNA can tolerate, and the outer boundary is set by the minimum required UV flux for biochemical reactions. Guo et al. find that only stars between about 0.6 and 1.9 Ms will be within both the traditional CHZ and the UV-CHZ.

Gonzalez finds that the prevalence of solar flares and other radiation emitted by a star can impact habitability, as can the frequency of asteroid impacts. All of these determine whether a star has a "circumstellar habitable zone," and whether it's large enough to make it likely that a planet will reside in it.

Then there's the galactic habitable zone (GHZ) which, according to Gonzalez, "describes the regions of the Milky Way most likely to contain habitable planetary systems." According to his paper, "Two classes of processes set its boundaries: the formation of Earth-like planets and threats to life." He continues:

[C]hemically-based life is not possible in the very early Universe before atoms formed or in the distant future, after all the stars burn out. Other considerations indicate that the boundaries of the CHA are much narrower than these extreme limits.

Another habitability constraint is the cosmic habitable age (CHA), which Gonzalez notes "is not a spatial zone, but rather, a temporal zone of habitability over the course of the evolution of the Universe." He finds that more research is needed to refine these boundaries.

So, based on our current knowledge of exoplanets, how common are habitable planets? This is a difficult question to answer precisely at present, but Gonzalez notes that known exoplanets suggest our solar system is unique:

Following the discoveries of the first few exoplanets (using the Doppler method), it quickly became apparent that exoplanetary systems do not resemble the Solar System; this is also true when detection biases are taken into account. Most systems either have planets in very short period orbits ("hot Jupiters") or planets in longer period eccentric orbits. ... It quickly became apparent following the discovery of the first exoplanet around a sun-like star (51 Pegasi) that exoplanetary systems are generally not like the Solar System and that additional processes not previously thought to be important in the Solar System must be invoked to explain the new observed trends.

In light of all this, consider again NASA's suggestion in the media that all we need for a habitable planet is liquid water -- i.e., the right distance from the host star. Gonzalez's article concludes that a lot more is required:

The ultimate question that most astrobiologists are seeking to answer is something like, "What is the probability that there are other planets with life?" The answer must incorporate the complete history of the Universe, including galaxy, star and planet formation and evolution. It is becoming clear that cosmology is not irrelevant to the formation and continued existence of habitable planets. The most basic elemental ingredients of planetary systems come from stars. Stars form and die in a galactic context, and galaxies form, interact and evolve in a cosmological context. At every scale, stochastic processes shape planetary systems, and they must be modeled with Monte Carlo methods. Processes with stochastic aspects occurring on the surface of a planet include volcanic eruptions, tectonics and climate. Asteroid and comet impacts can be triggered by planetary perturbations and nearby stellar and giant molecular cloud encounters. The location and timing of specific supernovae and gamma ray bursts cannot be predicted. Encounters between galaxies can trigger star formation, threatening already-formed planets and spawning new ones. ... The host star is the primary gravitational influence on the planets and affects every aspect of the dynamics of every body in a system, as well as tidal influences on the inner planets. Its electromagnetic spectrum has various effects on the atmospheres of the planets, and its particle radiation can influence atmospheric chemistry. Change one aspect of a habitable planetary system to make it non-habitable, and it might not be possible to make it habitable again with a single change to a different parameter.

Scientists searching for planets where E.T. may live have homed in on places where liquid water could exist. But a livable world may require not just water, but oceans, a new study suggests. ... Scientists calculate that as many as one in five sun-like stars may harbor an Earth-like planet in the so-called "Goldilocks" zone -- a region around a star thought to be just right for liquid water to exist. Researchers are now taking that a step further, looking for water and other signs of alien life in a planet's atmosphere.

The problem is that sitting within the CHZ doesn't guarantee that the planet will have an ocean:

Many planets lie in the so-called habitable zone of their stars, but without oceans, the surface temperatures fluctuate wildly, the researchers said. Mars is a good example, because the planet orbits within the sun's habitable zone, but has temperatures that vary by 180 degrees Fahrenheit (82 degrees Celsius) over the course of a Martian day.

This suggests that even if one in five sun-like stars has a planet orbiting at the right distance, there's no guarantee that liquid water will exist in sufficient quantities.

Indeed, as noted, even if the planet is within the CHZ as defined by distance from the host star, the existence of liquid water is another matter entirely. Getting sufficient water on a terrestrial planet like Earth is a challenge that requires more fine-tuning. Gonzalez explains in his paper in Life:

Water content is another very important requirement for habitability (neither too much nor too little). In addition to direct dynamical influences on the terrestrial planets, the Jovian planets also influenced the delivery of water to them. The Earth formed in a region of the early Solar System that was very dry, as evidenced by the enstatite chondrite meteorites (representative of the source bodies in the terrestrial planet region). Yet, Earth's water content is today estimated to be significantly greater than its formation at 1 AU would imply. The leading theories for the origin of Earth's water and other volatiles involve their delivery to Earth from more volatile-rich regions of the Solar System. Water delivery to Earth from comets, once a popular idea, can only account for about 10% of its crustal water inventory. In the classical, pre-Grand Tack scenario, consensus delivery of volatiles from the bodies in the outer asteroid belt, perturbed by Jupiter (with a more circular orbit than present), would have been too efficient, while an orbit for Jupiter with an eccentricity comparable to the present one would have left the Earth too dry. The delivery of volatiles to Earth form the outer asteroid belt region within the context of the Grand Tack scenario, however, is consistent with the measured geochemical constraints. The timing of the accretion of water and other volatiles by the Earth is such that it would have occurred while it was still growing in size, but accelerating towards the final stages for its formation.

All of this gives you an idea of the many factors which must be considered when assessing the prevalence of habitable planets. NASA's public rhetoric suggests that only one factor matters -- whether the planet is in the right region (the CHZ) to allow liquid water. Clearly, that is a gross simplification of the problem.

Gonzalez's paper in Life is forthright: "The considerations outlined above prevent us from estimating the probability of habitable planet formation using only analytic methods or by treating a planet in orbit around a star in isolation from the rest of the Universe" and "The traditional definition of the CHZ, based on the radiant energy from the host star, is outdated and should be replaced with a definition that also includes such considerations as planetary impact rate, orbital dynamical stability and episodic reductions in the size of the astrosphere."

He concludes, "A planetary system cannot be isolated from its broader galactic context when considering its formation and evolution in relation to habitability. A broader and more complete understanding of habitability requires merging the CHZ and GHZ concepts." After all, "Habitability factors are often interconnected in a complex web, and some factors can have multiple distinct effects on the habitability of a planetary system."

Orbiting at a distance that's "just right" from a star is just one small part of that complex web. Anxieties at NASA about meeting budgetary requirements don't change that fact.